When a hand-held scanner that reads an original image as it is being freely scanned across the surface of the original is used, it usually functions such that scanning positions; of an image sensor on the original are detected sequentially and image data is stored into an image memory on the basis of the scanning positions being detected.
An example of a method of detecting scanning positions of an image sensor on an original is disclosed in U.S. Pat. No. 4,260,979. This method uses a sheet with a reference grid printed on it as auxiliary equipment. This sheet is placed on top of an original, and is scanned with a photosensing element that is integrated with an image sensor for detecting the reference grid. A position of image data relative to the reference grid then is calculated from an output of the photosensing element.
Another method is disclosed, for example, in U.S. Pat. No. 4,581,761, which uses a tablet as auxiliary equipment. An original placed on top of the tablet, and is scanned with a detection coil that is integrated with an image sensor, A coordinate position on the tablet is detected from an output of the detection coil, and a position of the image data is calculated based on the coordinate position being detected.
Yet another method is disclosed, for example, in Laid-open Japanese Patent Application No. (Tokkai Sho) 62-15964, in which an encoder is installed with a wheel that moves together with an image sensor and generates a pulse in accordance with the wheel rotation. A position of the image sensor is detected from an output of the encoder, and a position of image data is calculated on the basis of the detected position.
When an image processing apparatus which applies one of these conventional detection method is used, a coordinate position of image data is calculated based on a scanning position being detected and stored into a predetermined address of an image memory. As a result, this apparatus can be prevented from being influenced by unintentional hand movement or meandering caused by a hand-held scanning, so that image data being stored as storage data in the image memory does not show any distortion.
However, the conventional system mentioned above has the following problems.
When auxiliary equipment, such as a sheet with a reference grid printed on it, or a tablet is used, it is necessary to process with accuracy and to install an expensive adjustment circuit for enhancing accuracy of a scanning position being detected and reducing an absolute position error, so that the cost usually becomes high. In addition, when auxiliary equipment with low accuracy is used, the scanning position being detected shows a large degree of absolute position error.
On the other hand, when an encoder installed with a wheel that moves together with an image sensor and generates pulses in accordance with wheel rotation is used, a scanning position is calculated by counting pulses of the encoder that are detected sequentially with a starting position of scanning as a reference point. As a result, a large degree of cumulative position error occurs due to low machine accuracy and slipping of the wheel.
When an absolute position error or a cumulative position error of a scanning position being detected reaches a large degree, a large position error arises with a normal scanning position on an original that is scanned by an image sensor.
When a coordinate position of image data is calculated based on this scanning position having a position error and is then stored into a predetermined address in an image memory, images which are reproduced on the image memory show great distortion. The distortion of reproduction images caused by this position error will be explained with reference to FIGS. 19B-19D.
FIG. 19B shows a hand-held scanner 1004 which scans freely on the surface of an original 1000. As shown in FIG. 19C, a prior hand-held scanner which is capable of scanning only one direction can not read a large size area on the original 1000. However, since image data is stored on an image memory only with an area being read by a sensor for one direction, a connection gap does not occur. FIG. 19D shows a case in which a hand-held scanner which reads a large size area on the original 1000 by freely scanning the original. In this case, when image data is stored in an image memory based on a scanning position having a position error, connected parts of the image cause a position error, so that the image is distorted. In particular, when a scanner causing a cumulative position error is used, a position error becomes worse as a scanning area broadens. Thus, it is no longer possible to make out the image itself.
An image distortion caused by a position error can be suppressed by shifting storage positions of image data in direction of X or Y axis as shown in FIG. 22, so that a good image without a connection error can be generated as shown in FIG. 19E.
This process, however, may raise the following problem: As shown in FIG. 22, cells in which no image data is stored may appear due to the process of shifting storage positions of image data (an area of cells in which no image data is stored is hereinafter referred to as a "map missing area"). A map missing area may be a conspicuous white line in a black image, and it degrades the image badly. If a map missing area appears in a character image as shown in FIG. 23, quality of the character image is reduced. A further problem is that an operator is unable to check promptly for the appearance of a map missing area.
Another problem in the current image processing apparatus is that a map missing area may appear when a free scanning is done such that its pixel density exceeds a permitted limit.
The operator cannot check for an abnormal state of the image quality in current image processing devices without performing an operation such as displaying or printing image after image data is stored into the image memory. In this case, the original image must be read again if an abnormal condition has occurred, which reduces the operation efficiency.